Time-multiplexing of MRI signals from different slices typically is called Simultaneous Echo Refocusing (SER) or Simultaneous Image Refocusing (SIR) EPI. Examples are described in (i) U.S. Pat. No. 6,614,225, (ii) Feinberg, D S, Reese T G, Wedeen V J, Simultaneous Echo Refocusing in EPI, Magn Res Med 48(1): 1-5 (2002), and (iii) Reese T G, Benner T, Wang R, Feinberg D A, and Wedeen V J, Halving Imaging Time of Whole Brain Diffusion Spectrum Imaging and Diffusion Tractography Using Simultaneous Image Refocusing in EPI, J Magn Res Imaging 29:517-522 (2009). The patent and the two articles cited in this paragraph are hereby incorporated by reference in this patent specification, as is every other patent and article cited elsewhere in this patent specification. FIG. 5 illustrates an example of a classical EPI pulse sequence, and FIG. 6 illustrates an example of a 2-slice SER pulse sequence, in each case showing pulses and time spacing that are not to scale. In the classical EPI example of FIG. 5, the 90° RF (radio frequency) pulse on the RF axis excites a slice in the body of a subject (e.g., a patient) in an imaging volume of an MRI scanner, according to the gradient pulses shown on the Gs axis. A train of MRI signals 1, 2, 3, . . . is emitted from the subject and read out with the help of a sequence of rephrasing gradient pulses, alternating in polarity, illustrated on the Gr axis and phase encoding gradient pulses illustrates on the Gp axis. The MRI signals are detected with one or more RF receiver coils, and the coil outputs are computer-processed to produce MRI image data for a k-space matrix and thereafter to produce an MRI image of the slice. In the 2-slice SIR example of FIG. 6, two RF pulses RFa and RFb are applied to the subject in time succession and, with the help of the gradient and 180° RF pulses illustrated in the preparatory time period Tpre, cause the patient to emit a time succession of N sets of two MRI signals (a) and (b) each. The first set comprises, in time sequence, MRI signal b1 and a1 for respective slices Sb and Sa of the subject, obtained in a single read interval N1; the second set comprises MRI signals a2 and b2 for the same two slices but in reverse order, obtained in a second read interval N2; the third set comprises signals b3 and a3, obtained in a read interval N3, etc. A rephrasing gradient shown on the gradient axis Gr alternates in polarity from one read interval N to the next, to thereby produce MRI signals for a total on N read intervals from the two RF pulses RFa and RFb, where N≧2. The two RF excitation pulses typically are 90° pulses that are slightly offset in frequency from each other. As a result, MRI echo signals are acquired from two slices in the time that a single echo MRI signal would be acquired absent the use of SIR. Thus, time-multiplexing of images in the readout periods N of SIR EPI increases data acquisition efficiency to thereby reduce average scan time, especially in diffusion imaging.
SIR data acquisition is impacted by the ratio of the preparatory time Tpre to the total MRI echo signal time. In one example, in an MRI data acquisition known as HARDI acquisition, the preparatory period Tpre is approximately 80 ms while the echo train is approximately 20 ms. The sharing of Tpre with two or more slices creates a large gain in sequence efficiency, defined here as net time of analog-to-digital (ADC) signal encoding per total sequence time. Another gain in efficiency in SIR is by the sharing of the many gradient switchings Tsw. Therefore scanners with slower slew rate or gradient ramp-time in their gradient systems (longer Tsw) also become more efficient with SIR, and similarly for lower resolution imaging (shorter repetition time TR relative to Tsw) efficiency and time savings increase. Despite the overall advantages of scan time reduction of SIR, the sampling time and echo spacing for each SIR slice are longer than for a classical EPI. The lengthening of the SIR echo train in the presence of local T2* (time constant for loss of phase coherence among spins oriented at an angle to the static magnetic field due to a combination of magnetic field inhomogeneities and the spin-spin relaxation) and Bo (static magnetic field) inhomogeneity increase image distortions to varying degrees but without losses in SNR (signal-to-noise ratio) provided the TE is unchanged in SIR EPI from classical EPI. In conditions requiring a minimum obtainable TE (echo time, or the time between the application of the 90° pulse and the peak of the echo signal in EPI) as in optimized diffusion imaging, the minimum TE of SIR is affected by the additional time of applying multiple excitation pulses plus the lengthened ADC read periods, and up to 10% SNR reduction has been found in SIR EPI. Using SIR in fMRI (functional MRI), there is no penalty in SNR as TE is typically lengthened from the minimally obtainable TE since BOLD (blood-oxygen level dependent) contrast is optimized when TE=T2*.
A second approach, independent of SIR, involves frequency-multiplexing of images by combining excitation of slices at different off-resonance frequencies with subsequent de-multiplexing based on spatial sensitivity differences of RF receiver coils, a technique referred to as Multi-Band (MB) excitation. Examples of the MB approach are described in (i) Moller S, Auerbach E, van de Moortele P F, Adriany G, Ugurbil K, fMRI with 16-Fold reduction using multibanded multislice sampling, Proc. Int. Soc. Magn. Reson. In Med., 2008. 16: p. 2366, (ii) Moller S, Yacoub E, Olman C A, Auerbach E, Strupp J, Harel N, and Ugurbil K, Multiband Multislice GE-EPI at 7 Tesla, With 16-Fold Acceleration Using Partial Parallel Imaging With Application to High Spatial and Temporal While-Brain FMRI, (in press when the provisional application was filed, with a copy of the paper attached hereto and incorporated by reference herein; the published version is Feinberg D A, Moeller S, Smith S M, Auerbach E, Ramanna S, Glasser M F, Miller K L, Ugurbil K, and Yacoub E, Multiplexed Echo Planar Imaging for Sub-Second Whole Mrain fMRI and Fast Diffusion Imaging, PLoS, December 2010, volume 5, issue 12, e15710, pages 1-11), and (iii) Larkman D J, Hajnal J V, Herlihy A H, Coutts G A, Young I R, Ehnholm G. Use of multicoil arrays for separation of signal from multiple slices simultaneously excited. J Magn Reson Imaging 2001; 13(2):313-317. In MB excitation, increased efficiency is achieved by exciting several slices simultaneously. The MRI signals from those slices are unfolded using spatial encoding information present in RF receiver systems. Each of the several receiver coils yields a combination of MRI signals from all excited slices weighted by the sensitivity of the respective coil. A matrix inversion can provide a solution to unfold these signals so as to reconstruct MR images of the respective slices.
The MB acquisition of multiple slices at one time accelerates the volume coverage by the number of bands used in an MB RF excitation pulse (and thus the number of simultaneously excited and read out slices, and also results in reduced gradient demands and consequent reduced levels of acoustic noise for an un-accelerated acquisition of the same number of slices in which each slice is acquired separately. An MB MRI data acquisition technique available in MRI scanners from Siemens under the name SENSE provides a solution to aliasing. The separation of the aliased slice signals requires a different reference acquisition for GRAPPA (another pulse sequence provided by Siemens), but not for SENSE which directly separates aliased voxels. The two reconstructions have been shown to perform equally for GE (FLASH) imaging, but with GRAPPA being more desirable for high-field EPI imaging. The data size is reduced by a factor equal to the number of bands as several slices are contained within one matrix. Compared to equivalent multi-slice acquisitions needed to achieve the same number of slices, the repetition time TR is reduced by this same factor, allowing a larger number of slice images (and thus a better characterization of the temporal dynamics) to be acquired over the same time. Lastly, since each slice is excited and sampled identically, there is no significant SNR loss due to reduced data collection as is encountered with conventional parallel imaging along the phase encode direction, where under-sampling is used to accelerate the acquisition. There can be, however, SNR losses associated with separation of aliased image slices.